This invention is related to a method of cladding tubing and manufacturing condenser cores. The method provides an enhanced bond between the substrate material and the cladding material without requiring complicated and expensive surface preparation processes prior to cladding.
In many applications, it is desirable to utilize metallic products that have an outer surface layer that consists of a different metallic material than the metallic substrate material used to manufacture the remainder of the product. This metallic outer surface layer may, for instance, inhibit corrosion, increase wear resistance, promote thermal transfer, or allow for later manufacturing operations, such as brazing or welding.
A number of methods are known for forming metallic products having a metallic outer surface layer that consists of a different metallic substance than the remainder of the product. In the method described in U.S. Pat. No. 5,172,476, for instance, a sheet of aluminum base material is bonded to a sheet of aluminum-silicone material. This bilaminate sheet material is then roll formed into a tubular shape and the seam where the original outer edges of the sheet material meet is welded to produce aluminum tubing having an aluminum-silicone outer surface layer.
A metallic outer surface layer can also be bonded to the products after the products have already been fabricated. A variety of methods for applying metallic outer surface layers over metallic substrate layers are known in the art.
Surface coating method techniques such as spraying, chemical vapor deposition, physical vapor deposition, and diffusion coating can be used to impart a metallic coating layer over a metallic substrate material. Creating a metallic surface coating by flame spraying is described in U.S. Pat. No. 5,133,126. The method described in this patent can produce aluminum tubing that is covered by an anticorrosive zinc outer surface layer. Creating a metallic surface coating by arc spraying is described in U.S. Pat. No. 4,753,849. The method described in this patent can produce copper tubing having a zinc/copper pseudo alloy outer surface layer. Metallic surface layers can also be provided over a metallic substrate material by electroplating, dip coating, galvanizing and similar techniques.
Another method for providing a product with a metallic outer surface layer is referred to as cladding. In a cladding process, metallic cladding material in solid form is brought into contact with the outer surface of the product and pressure is applied which solid-state bonds the metallic cladding material to the metallic substrate material. Several methods for cladding metallic products are known.
In the method described in U.S. Pat. No. 3,616,982, a wire-like core material passes through an annular shaving die to expose a virgin metal surface and then passes through a manifold having a protective or oxide-reducing atmosphere. Cladding material is brought into contact with the core material and peripherally grooved rolls squeeze the cladding material against the core material and bond the cladding to the core entirely around the periphery of the core.
In the method described in U.S. Pat. No. 3,389,455, tubing is heated as it passes through a retort that has an inert or reducing atmosphere. Powderized metallic cladding materials are then brought into contact with the tubing and the materials are squeezed and compressed. The squeezed and compressed assembly is then sintered to complete the cladding process.
In the method described in U.S. Pat. No. 5,056,209, an inner pipe (i.e. the metallic substrate material) is placed within an outer pipe (i.e. the metallic cladding material) to produce a combined billet. The billet is then heated and subjected to a hot extrusion process. The hot extrusion process deforms the billet and bonds the metallic cladding material to the metallic substrate material.
A problem inherent in those cladding methods which clad a product after the product has already been fabricated is that many metallic materials can form tenacious surface oxide layers when exposed to the atmosphere or other oxidizing environments. Other materials, such as adsorbed gas films or lubricant residues, can also contaminate the outer surface of the product between the product fabrication process and the cladding process. These contaminates can prevent the formation of interatomic bonds that are essential to obtaining a sufficient solid-state bond between the metallic substrate material and the metallic cladding material during the cladding process.
To reduce the quantity of oxides and other contaminates present on the outside of the products prior to the cladding process, other prior art cladding methods employ complicated and expensive surface treatment methods in an attempt to assure that substantial fresh material is present on the outer surface of the product when the metallic substrate material and the metallic cladding material are brought into contact. In these methods, the surface treatment process may consist, for instance, of heating the product in an inert or reducing atmosphere; shaving, brushing, or scraping the outer surface of the product; or treating the outer surface of the product with degreasing solvents or an aqueous sodium hydroxide solution. Each of these methods are intended to increase the amount of fresh metallic substrate material surface present when the metallic cladding material is brought into contact with the metallic substrate material. As seen in the above mentioned '982 patent, a virgin metal surface is preferred. The application of an antioxidizing compound to the metallic substrate material, such as the application of a oxidation preventing petroleum-based coating, can successfully inhibit the formation of the surface oxide layers. This antioxidizing compound, however, then itself becomes a contaminate that inhibits the proper bonding between the metallic substrate material and the metallic cladding material unless it is completely removed prior to the cladding process.
Some cladding methods effectively disrupt the contaminated outer surface layer by deforming the product after the cladding material has been brought into contact with the product. In cladding methods that employ powdered metallic cladding materials, such as described in U.S. Pat. No. 3,389,455, the particles of the powder will tend to bite into the surface of the metallic substrate material, which will help to break through the thin oxide film formed on the outside of the product. In cladding methods that employ a hot extrusion process after the cladding material has been placed into contact with the substrate material, such as described in U.S. Pat. No. 5,056,209, the original outer surface of the substrate material is stretched as it is deformed. This produces a substantial amount of fresh material surface to which the cladding material can effectively bond.
The quantity of contaminates present on the outside surfaces of the products will depend, of course, on the amount of time and types of conditions the products were subjected to after they were fabricated. To assure that a proper material surface is present, it is typically necessary to "overprepare" the outer surface of the products to be relatively certain that any quantity of contaminates that could be reasonably expected to be present on the outer surface of the products has been thoroughly removed. These surface preparation processes often introduce significant expense to the overall manufacturing cost of the products.
An improved method for manufacturing clad extruded products is therefore desirable to assure that a strong consistent bond is obtained between the metallic substrate material and the metallic cladding material and to eliminate the need for complicated and expensive surface preparation treatments prior to the cladding process.
The present inventive method for manufacturing clad extruded metallic products takes advantage of the fact that when metallic products are extruded using an unlubricated extrusion process, a dead-metal zone is created and material flow takes place by shearing the material along the surface of this zone. The extruded product thereby acquires a completely freshly formed exterior surface which is free of oxides, lubricants, absorbed gas films, and other contaminates. This freshly formed surface offers an ideally prepared surface for a subsequent cladding process. If the metallic cladding material is brought into intimate contact with the freshly formed surface on the outside of the extrudate promptly after the extrudate has been discharged from the extrusion die, no cleaning or subsequent surface preparation treatment of the exterior of the product is required.
By cladding the products promptly after the hot extrusion process, a bond can be produced between the metallic substrate material and the metallic cladding material and the need for complicated and expensive surface preparation treatments prior to the cladding process is eliminated.
A further aspect of this inventive method is the manufacturing of condensor cores. In this process, the cladding is applied to an extruded tube as outlined above and then cooled to room temperature. Fin stock is then positioned in close-fit engagement about the cladded tubing and the cladding material itself is utilized as the brazing/soldering material, depending on the securement method. For example, the initially assembled fin stock and clad tubing can be provided into a brazing furnace where the cladding material is drawn by capillary action into the joint gap to form the fillet. In a soldering process, the cladding material is utilized as the solder.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.